0001746473-21-000008 | 10-Q | XBRL Viewer

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Subsequent Events	3 Months Ended
Subsequent Events	Mar. 31, 2021
Subsequent Events [Abstract]
Subsequent Events	17. Subsequent Events The Company has evaluated subsequent events for financial statement purposes occurring through May 10, 2021, the date these financial statements were issued.

Equity Incentive Plans and Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - Restricted stock awards - $ / shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding and unvested, beginning balance (in shares)	126,522
Issued (in shares)	0	0
Vested (in shares)	(30,277)
Repurchases (in shares)	0
Outstanding and unvested, ending balance (in shares)	96,245
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Weighted-Average Grant Date fair value, Outstanding and unvested, beginning balance (USD per share)	$ 1.85
Weighted-Average Grant Date fair value, Issued (USD per share)	0
Weighted-Average Grant Date fair value, Vested (USD per share)	1.69
Weighted-Average Grant Date fair value, Repurchases (USD per share)	0
Weighted-Average Grant Date fair value, Outstanding and unvested, ending balance (USD per share)	$ 1.90

Common Stock - Narrative (Details) - $ / shares	3 Months Ended	70 Months Ended
Common Stock - Narrative (Details) - $ / shares	Mar. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Equity [Abstract]
Common stock, shares authorized (in shares)	300,000,000	300,000,000	300,000,000
Common stock, par value (USD per share)	$ 0.0001	$ 0.0001	$ 0.0001
Common stock, voting rights	The holders of shares of common stock are entitled to one vote for each share of common stock held at any meeting of stockholders and at the time of any written action in lieu of a meeting.
Dividends declared (USD per share)		0
Dividends paid (USD per share)		$ 0

Defined Contribution Plan	3 Months Ended
Defined Contribution Plan	Mar. 31, 2021
Retirement Benefits [Abstract]
Defined Contribution Plan	15. Defined Contribution Plan The Company sponsors a defined contribution plan under Section 401(k) of the Internal Revenue Code covering substantially all full-time U.S. employees. Employee contributions are voluntary and are determined on an individual basis subject to the maximum allowable under federal tax regulations. The Company made contributions to the plan of $0.2 million for the three months ended March 31, 2021 and 2020.

Equity Incentive Plans and Stock-Based Compensation - 2020 Employee Stock Purchase Plan - Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for 2020 Employee Stock Purchase Plan (Details) - 2020 Employee Stock Purchase Plan	3 Months Ended
	Mar. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.07%
Expected term of options (in years)	6 months
Expected stock price volatility	89.51%
Expected dividend	0.00%

Accrued Liabilities and Other Long-Term Liabilities - Summary of Accrued Liabilities (Details) - USD ($)
$ in Thousands

Mar. 31, 2021

Dec. 31, 2020

Accrued Liabilities and Other Liabilities [Abstract]

Accrued compensation and benefits

$ 2,300

$ 4,542

Accrued research and development expenses

3,996

3,274

Other accrued liabilities

965

1,675

Deferred rent

103

Total accrued liabilities

$ 7,364

$ 9,576

^[1]

[1]	The condensed balance sheet as of December 31, 2020 has been derived from the audited financial statements as of that date.

Equity Incentive Plans and Stock-Based Compensation

3 Months Ended

Mar. 31, 2021

Share-based Payment Arrangement [Abstract]

Equity Incentive Plans and Stock-Based Compensation

11. Equity Incentive Plans and Stock-Based Compensation

In August 2015, the Company's board of directors adopted the 2015 Equity Incentive Plan (as amended, the “2015 Plan”), which provides for the grant of incentive stock options, nonqualified stock options or other awards including stock appreciation rights and restricted stock awards to the Company’s employees, officers, directors, advisors, and consultants for the purchase of up to 1.5 million shares of the Company’s common stock. In July 2018 and January 2019, the 2015 Plan was amended to increase the number of shares reserved thereunder by 1.0 million and 0.4 million shares, respectively. In March 2020, the Company’s board of directors and stockholders voted to increase the number of shares reserved for issuance under the 2015 Plan by 1.4 million shares. In May 2020, the board of directors adopted the 2020 Stock Options and Incentive Plan (the “2020 Plan”). The 2015 Plan was suspended and no further grants may be issued under the 2015 Plan.

The 2020 Plan provides for the grant of incentive stock options, nonqualified stock options or other awards including stock appreciation rights, restricted stock awards and restricted stock units to the Company’s employees, officers, directors, advisors, and consultants for the purchase of up to 4.2 million shares of the Company’s common stock. In addition, to the extent that awards outstanding under the 2020 Plan or the 2015 Plan are cancelled, forfeited or held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without any issuance of stock, expire or are otherwise terminated (other than by exercise) subsequent to May 2020, the shares of common stock reserved for issuance pursuant to such awards will become available for issuance as shares of common stock under the 2020 Plan. The 2020 Plan provides that an additional number of shares will automatically be added to the shares authorized for issuance under the 2020 Plan on January 1 of each year beginning January 1, 2021. The number of shares added each January 1 will be equal to the lesser of: (i) 5% of the outstanding shares on the immediately preceding December 31 or (ii) such amount as determined by the administrator of the 2020 Plan, which is the compensation committee of the board of directors of the Company. As of March 31, 2021, 4.3 million shares remained available for issuance under the 2020 Plan.

Prior to the adoption of the 2020 Plan, options under the 2015 Plan could be granted for periods of up to 10 years and at prices no less than 100% of the estimated fair value of the shares on the date of grant as determined by our board of directors, provided, however, that the exercise price of an incentive stock option granted to a person owning (or deemed to own) stock possessing more than ten percent of the total combined voting power of all classes of stock of the Company or any affiliate of the Company (a "10% shareholder") could not be less than 110% of the estimated fair value of the shares on the date of grant and the option was not exercisable after the expiration of five years from the date of grant.

Options under the 2020 Plan may be granted for periods of up to 10 years and at prices no less than the market price of the Company’s common stock on the date of grant, provided, however, that the exercise price of an incentive stock option granted to a 10.0% shareholder shall not be less than 110.0% of the estimated fair value of the shares on the date of grant and the option is not exercisable after the expiration of five years from the date of grant.

Restricted Common Stock Awards

The Company granted restricted stock awards under the 2015 Plan. The purchase price of the restricted common stock awards was the estimated fair value as determined by the Company's board of directors at the issuance date. The shares vest from one to four years and vesting could be accelerated upon a change in control. A holder of an award may pay a total purchase price or a part of the purchase price for granted shares at any time during the vesting periods. Upon termination of employment, the Company has the right to repurchase any unvested restricted shares. The repurchase price for unvested shares of common stock will be the lower of (i) the fair market value on the date of repurchase or (ii) their original purchase price.

During the vesting term, holders of restricted stock awards are deemed to be common stock shareholders and have dividends and voting rights.

The Company accounted for restricted stock awards as early exercised options and recognized a liability in other liabilities when cash was received for the purchase of shares of restricted stock. As shares of restricted stock vested, the Company reclassified the liability to common stock and additional paid in capital. As of March 31, 2021 and December 31, 2020, the Company recorded a liability included in accrued expenses and other liabilities of $8,000 and $10,000, respectively.

There were no grants of restricted stock awards for the three months ended March 31, 2021 and 2020.

The following table summarizes restricted stock activity during the three months ended March 31, 2021:


	Number of Shares	Weighted- Average Grant Date fair value
Outstanding and unvested, as of December 31, 2020	126,522	$	1.85
Issued	—	$	—
Vested	(30,277)	$	1.69
Repurchases	—	$	—
Outstanding and unvested, as of March 31, 2021	96,245	$	1.90

Restricted stock awards of 4,195 shares with a weighted-average grant date fair value of $0.08 per share, were not purchased by the award holders as of March 31, 2021. As these shares of the restricted common stock awards were not issued, they are not included in the table above.

The aggregate fair value of restricted stock awards vested during the three months ended March 31, 2021 was $51,000. Total intrinsic value of outstanding unvested restricted stock awards as of March 31, 2021 was $2.2 million.

Incentive Stock Options and Nonqualified Stock Options

Stock options issued under either the 2015 Plan or the 2020 Plan generally vest over four years and expire ten years from the date of grant. Certain options provide for accelerated vesting if there is a change in control, as defined in the respective plans.

The Company used Black-Scholes option pricing model to estimate stock-based compensation expense for stock option awards with the following assumptions:


	Three Months Ended March 31,
	2021	2020
Expected volatility	74.95% - 75.72%	72.10% - 77.50%
Risk-free interest rate	0.61% - 1.06%	0.52% - 0.82%
Expected dividend	—	—
Expected term (in years)	5.77 - 6.08	5.38 - 6.75
Underlying common stock fair value	26.13 - 38.23	6.22

The Company granted 1,082,053 stock options under the 2020 Plan during the three months ended March 31, 2021.

A summary of option activity under the 2015 Plan and the 2020 Plan is as follows:


	Number of Options	Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (in Years)	Aggregate Intrinsic Value
Outstanding as of December 31, 2020	2,993,855	$	8.18	8.84	$	43,890
Granted	1,082,053	$	27.11
Exercised	(105,734)	$	5.28
Forfeited	(41,887)	$	7.59
Outstanding as of March 31, 2021	3,928,287	$	13.30	8.97	$	100,437
Exercisable as of March 31, 2021	856,283	$	5.24	8.31	$	29,194
Vested and expected to vest as of March 31, 2021	3,928,287	$	13.30	8.97	$	100,437

Aggregate intrinsic value represents the difference between the fair value of the underlying common stock and the exercise price as of March 31, 2021. The weighted-average grant date fair value of options granted during the three months ended March 31, 2021 was $17.62 per share.

Stock-Based Compensation Expense

The following table presents the components and classification of stock-based compensation expense for the Company’s stock-based awards for the three months ended March 31, 2021 and 2020 (in thousands):


	Three Months Ended March 31,
	2021		2020
Restricted stock awards	$	51	$	57
Stock options and ESPP	2,527		368
Total stock-based compensation expense	$	2,578	$	425
Research and development expenses	$	1,170	$	164
General and administrative expenses	$	1,408	$	261

As of March 31, 2021, there was $0.2 million of unrecognized compensation costs that is expected to be recognized over the weighted-average periods of 1.00 year related to restricted stock awards. As of March 31, 2021, there was $30.0 million of unrecognized compensation costs that is expected to be recognized over the weighted-average periods of 3.02 years related to stock options.

2020 Employee Stock Purchase Plan

In June 2020, the Company adopted the Company's 2020 Employee Stock Purchase Plan (the "2020 ESPP"). The Company reserved 700,000 shares of common stock for future issuance under the plan. The 2020 ESPP provides that the number of shares reserved and available for issuance will automatically increase on January 1 of each calendar year, beginning January 1, 2021, by the least of (1) 1.0% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, (2) 700,000 shares or (3) such lesser amount as determined by the administrator of the 2020 ESPP, which is the compensation committee of the board of directors of the Company.

Under the 2020 ESPP, eligible employees may purchase shares of our common stock through payroll deductions that cannot exceed 15% of each employee’s salary. The 2020 ESPP provides for a six-month offering period. At the end of the purchase period, eligible employees are permitted to purchase shares of common stock at the lower of 85% of the fair market value at the beginning of the offering period or 85% of the fair market value at the end of the purchase period, subject to tax limitations on the total value of the purchase. The 2020 ESPP is considered a compensatory plan, and the Company recorded $0.3 million in stock-based compensation expense for the three months ended March 31, 2021. There was no stock-based compensation expense attributed to the 2020 ESPP for the three months ended March 31, 2020 as the 2020 ESPP was adopted in August 2020. As of March 31, 2021, 59,766 shares of common stock were issued under the 2020 ESPP. The Company used Black-Scholes option pricing model to estimate stock-based compensation expense for the 2020 ESPP with the following assumptions:


	Three Months Ended March 31,
	2021
Risk-free interest rate	0.07	%
Expected term of options (in years)	0.5
Expected stock price volatility	89.51	%
Expected dividends	—

Property and Equipment, net - Schedule of Property and Equipment (Details) - USD ($)
$ in Thousands

Mar. 31, 2021

Dec. 31, 2020

Property, Plant and Equipment [Line Items]

Total property and equipment, gross

$ 8,113

$ 7,809

Less: Accumulated depreciation

(3,847)

(3,488)

Total property and equipment, net

4,266

4,321

^[1]

Computer equipment and software

Property, Plant and Equipment [Line Items]

Total property and equipment, gross

Laboratory equipment

Property, Plant and Equipment [Line Items]

Total property and equipment, gross

6,921

6,540

Leasehold improvements

Property, Plant and Equipment [Line Items]

Total property and equipment, gross

947

Construction-in-progress

Property, Plant and Equipment [Line Items]

Total property and equipment, gross

$ 223

$ 300

[1]	The condensed balance sheet as of December 31, 2020 has been derived from the audited financial statements as of that date.

Adimab Development and Option Agreement (the "Adimab Agreement")	3 Months Ended
	Mar. 31, 2021
Research and Development [Abstract]
Adimab Development and Option Agreement (the "Adimab Agreement")	Adimab Development and Option Agreement (the "Adimab Agreement") In 2018, we entered into a development and option agreement with Adimab, LLC (“Adimab”) for the discovery and optimization of proprietary antibodies as potential therapeutic product candidates. Under the Adimab Agreement, we will select biological targets against which Adimab will use its proprietary platform technology to research and develop antibody proteins using a mutually agreed upon research plan. We are required to pay Adimab an agreed upon rate for its full-time employees during the discovery period while Adimab performs research on each target under the applicable research plan. We have an exclusive option to obtain a worldwide, royalty-bearing, sublicensable license under Adimab platform patents and other Adimab technology to research, develop and commercialize up to 24 antibodies of our selection. As of March 31, 2021, we have not exercised our option to further develop any antibody proteins. During the three months ended March 31, 2021 we recognized research and development expenses under the Adimab Agreement of $28,000 related to full-time employee costs. No costs were recognized during the three months ended March 31, 2020.

Equity Incentive Plans and Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2021

Share-based Payment Arrangement [Abstract]

Summary of Restricted Stock Activity

The following table summarizes restricted stock activity during the three months ended March 31, 2021:


	Number of Shares	Weighted- Average Grant Date fair value
Outstanding and unvested, as of December 31, 2020	126,522	$	1.85
Issued	—	$	—
Vested	(30,277)	$	1.69
Repurchases	—	$	—
Outstanding and unvested, as of March 31, 2021	96,245	$	1.90

Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for Stock Option Awards

The Company used Black-Scholes option pricing model to estimate stock-based compensation expense for stock option awards with the following assumptions:


	Three Months Ended March 31,
	2021	2020
Expected volatility	74.95% - 75.72%	72.10% - 77.50%
Risk-free interest rate	0.61% - 1.06%	0.52% - 0.82%
Expected dividend	—	—
Expected term (in years)	5.77 - 6.08	5.38 - 6.75
Underlying common stock fair value	26.13 - 38.23	6.22

Summary of Option Activity Under the 2015 Plan and 2020 Plan

A summary of option activity under the 2015 Plan and the 2020 Plan is as follows:


	Number of Options	Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (in Years)	Aggregate Intrinsic Value
Outstanding as of December 31, 2020	2,993,855	$	8.18	8.84	$	43,890
Granted	1,082,053	$	27.11
Exercised	(105,734)	$	5.28
Forfeited	(41,887)	$	7.59
Outstanding as of March 31, 2021	3,928,287	$	13.30	8.97	$	100,437
Exercisable as of March 31, 2021	856,283	$	5.24	8.31	$	29,194
Vested and expected to vest as of March 31, 2021	3,928,287	$	13.30	8.97	$	100,437

Summary of Components and Classification of Stock-based Compensation Expense for Stock-based Awards


	Three Months Ended March 31,
	2021		2020
Restricted stock awards	$	51	$	57
Stock options and ESPP	2,527		368
Total stock-based compensation expense	$	2,578	$	425
Research and development expenses	$	1,170	$	164
General and administrative expenses	$	1,408	$	261

Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for 2020 Employee Stock Purchase Plan

The Company used Black-Scholes option pricing model to estimate stock-based compensation expense for the 2020 ESPP with the following assumptions:


	Three Months Ended March 31,
	2021
Risk-free interest rate	0.07	%
Expected term of options (in years)	0.5
Expected stock price volatility	89.51	%
Expected dividends	—

Equity Incentive Plans and Stock-Based Compensation - Restricted Common Stock Awards Narrative (Details) - Restricted stock awards - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during period (in shares)	0	0
Number of shares, not purchased (in shares)	4,195
Weighted-average grant date fair value, not purchased (USD per share)	$ 0.08
Aggregate fair value of restricted stock awards vested	$ 51
Total intrinsic value of outstanding unvested restricted stock awards	2,200
Accrued Expenses and Other Liabilities
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accrued expenses and other liabilities	$ 8		$ 10
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	1 year
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years

Accrued Liabilities and Other Long-Term Liabilities

3 Months Ended

Mar. 31, 2021

Accrued Liabilities and Other Liabilities [Abstract]

Accrued Liabilities and Other Long-Term Liabilities

5. Accrued Liabilities and Other Long-Term Liabilities

Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):


	As of March 31, 2021		As of December 31, 2020
Accrued compensation and benefits	$	2,300	$	4,542
Accrued research and development expenses	3,996		3,274
Other accrued liabilities	965		1,675
Deferred rent	103		85
Total accrued liabilities	$	7,364	$	9,576

Accrued compensation and benefits consist primarily of accrued bonuses and accrued vacation.

Other Long-Term Liabilities

Other long-term liabilities consisted of the following (in thousands):


	As of March 31, 2021		As of December 31, 2020
Deferred rent	$	571	$	581
Leasehold incentive obligation	263		283
Other liabilities — deposits	1		2
Total other long-term liabilities	$	835	$	866

Net (Loss)Income Per Share Attributable to Common Stockholders (Tables)

3 Months Ended

Mar. 31, 2021

Earnings Per Share [Abstract]

Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share

The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net (loss)/income per share attributable to common stockholders for the periods presented, because including them would have been antidilutive:


	Three Months Ended March 31,
	2021	2020
Redeemable convertible preferred stock (on an as-converted basis)	—	22,432,029
Options to purchase common stock	3,928,287	2,603,744
Restricted stock awards granted and not purchased	4,195	4,195
Unvested restricted shares	96,245	237,997
Total	4,028,727	25,277,965

Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Loss Per Share

A reconciliation of the numerator and denominator used in the calculation of the basic and diluted net loss per share attributable to common stockholders is as follows (in thousands, except share and per share amounts):


	Three Months Ended March 31,
	2021		2020
Net income (loss) per share:
Numerator
Net income (loss)	$	(22,856)	$	11,029
Less: undistributed earnings to preferred stockholders	—		(11,029)
Net loss attributable to common stockholders	$	(22,856)	$	—
Denominator
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders:
Basic	35,645,205		1,897,669
Diluted	35,645,205		1,897,669
Net loss per share attributable to common stockholders:
Basic	$	(0.64)	$	—
Diluted	$	(0.64)	$	—

Financial Instruments - Narrative (Details) - USD ($)	3 Months Ended
Financial Instruments - Narrative (Details) - USD ($)	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities measured at fair value	$ 0		$ 0
Transfers of assets between Level 1 and Level 2	0	$ 0
Transfers of assets between Level 2 and Level 1	0	0
Transfers of liabilities between Level 1 and Level 2	0	0
Transfers of liabilities between Level 2 to Level 1	0	0
Transfer out of Level 3 measurement	0	0
Transfer in to Level 3 measurement	0	$ 0
Fair Value Measurements, Non-recurring Basis
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities measured at fair value	0		0
Assets measured at fair value	$ 0		$ 0

Accrued Liabilities and Other Long-Term Liabilities (Tables)

3 Months Ended

Mar. 31, 2021

Accrued Liabilities and Other Liabilities [Abstract]

Schedule of Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):


	As of March 31, 2021		As of December 31, 2020
Accrued compensation and benefits	$	2,300	$	4,542
Accrued research and development expenses	3,996		3,274
Other accrued liabilities	965		1,675
Deferred rent	103		85
Total accrued liabilities	$	7,364	$	9,576

Summary of Other Long-Term Liabilities

Other long-term liabilities consisted of the following (in thousands):


	As of March 31, 2021		As of December 31, 2020
Deferred rent	$	571	$	581
Leasehold incentive obligation	263		283
Other liabilities — deposits	1		2
Total other long-term liabilities	$	835	$	866

Property and Equipment, net

3 Months Ended

Mar. 31, 2021

Property, Plant and Equipment [Abstract]

Property and Equipment, net

4. Property and Equipment, net

Property and equipment, net consisted of the following (in thousands):


	As of March 31, 2021		As of December 31, 2020
Computer equipment and software	$	22	$	22
Laboratory equipment	6,921		6,540
Leasehold improvements	947		947
Construction-in-progress	223		300
Total property and equipment, gross	8,113		7,809
Less: Accumulated depreciation	(3,847)		(3,488)
Total property and equipment, net	$	4,266	$	4,321

Depreciation expense for the three months ended March 31, 2021 was $0.4 million. Depreciation expense for the three months ended March 31, 2020 was $0.3 million.

Net (Loss)Income Per Share Attributable to Common Stockholders

3 Months Ended

Mar. 31, 2021

Earnings Per Share [Abstract]

Net (Loss)Income Per Share Attributable to Common Stockholders

16. Net (Loss)/Income Per Share Attributable to Common Stockholders


	Three Months Ended March 31,
	2021	2020
Redeemable convertible preferred stock (on an as-converted basis)	—	22,432,029
Options to purchase common stock	3,928,287	2,603,744
Restricted stock awards granted and not purchased	4,195	4,195
Unvested restricted shares	96,245	237,997
Total	4,028,727	25,277,965


	Three Months Ended March 31,
	2021		2020
Net income (loss) per share:
Numerator
Net income (loss)	$	(22,856)	$	11,029
Less: undistributed earnings to preferred stockholders	—		(11,029)
Net loss attributable to common stockholders	$	(22,856)	$	—
Denominator
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders:
Basic	35,645,205		1,897,669
Diluted	35,645,205		1,897,669
Net loss per share attributable to common stockholders:
Basic	$	(0.64)	$	—
Diluted	$	(0.64)	$	—

Commitments and Contingencies - Schedule of Future Minimum Payments (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Leases [Abstract]
2021 (remainder of the year)	$ 1,529
2022	2,098
2023	2,171
2024	2,247
2025 and beyond	1,143
Total	$ 9,188

Accrued Liabilities and Other Long-Term Liabilities - Summary of Other Long-Term Liabilities (Details) - USD ($)
$ in Thousands

Mar. 31, 2021

Dec. 31, 2020

Accrued Liabilities and Other Liabilities [Abstract]

Deferred rent

$ 571

$ 581

Leasehold incentive obligation

263

283

Other liabilities — deposits

Total other long-term liabilities

$ 835

$ 866

^[1]

[1]	The condensed balance sheet as of December 31, 2020 has been derived from the audited financial statements as of that date.

Condensed Balance Sheets (Unaudited) - USD ($)
$ in Thousands

Mar. 31, 2021

Dec. 31, 2020

^[1]

Current assets

Cash and cash equivalents

$ 50,819

$ 50,882

Short-term investments

213,281

226,012

Accounts receivable

2,174

9,279

Tax credit receivable

Prepaid expenses and other current assets

4,180

4,498

Total current assets

270,537

290,754

Property and equipment, net

4,266

4,321

Other non-current assets

451

Total assets

275,254

295,526

Current liabilities

Accounts payable

2,886

2,023

Accrued liabilities

7,364

9,576

Total current liabilities

10,250

11,599

Other long-term liabilities

835

866

Total liabilities

11,085

12,465

Stockholders’ equity

Common stock, $0.0001 par value; 300,000,000 shares authorized at March 31, 2021 and December 31, 2020; and 35,748,336 and 35,552,795 shares issued and outstanding at March 31, 2021 and December 31, 2020, respectively;

Additional paid-in capital

404,868

400,918

Accumulated deficit

(140,684)

(117,828)

Accumulated other comprehensive loss

(18)

(32)

Total stockholders’ equity

264,169

283,061

Total liabilities and stockholders’ equity

$ 275,254

$ 295,526

[1]	The condensed balance sheet as of December 31, 2020 has been derived from the audited financial statements as of that date.

Novartis Collaboration and License Agreement ("the Novartis Agreement") - Narrative (Details) - Novartis	1 Months Ended	3 Months Ended		12 Months Ended
	Oct. 31, 2019 USD ($) research stockClass	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Novartis Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of Integrin research targets \| stockClass	3
Proceeds from upfront non-refundable license fee				$ 50,000,000.0
Revenue recognized from related party		$ 2,200,000	$ 3,900,000
Novartis Agreement \| Achievement of First Patient Dosing Milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized from related party		$ 0	$ 25,000,000.0
Novartis Agreement, R&D For PLN-1474 \| Research and development expenses
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Transaction price of Novartis agreement				19,600,000
Novartis Agreement, R&D For Integrin Research Targets \| Research and development expenses
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Transaction price of Novartis agreement				13,400,000
Maximum \| Novartis Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of Integrin research targets \| research	3
Collaboration and license agreement, integrin research targets amount (up to)				$ 416,000,000.0
Amount remaining for achievement	$ 391,000,000.0

Financial Instruments - Summary of Cash and Cash Equivalents, Money Market Funds and Short-term Investments by Significant Investment Category (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost	$ 243,854	$ 263,729
Unrealized Gains	20	15
Unrealized Losses	(37)	(47)
Market Value	243,837	263,697
Level 1 \| Money Market Funds
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost	22,056	27,686
Unrealized Gains	0	0
Unrealized Losses	0	0
Market Value	22,056	27,686
Level 2 \| Corporate debt securities included in cash and cash equivalents and short-term investments
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost	160,133	118,759
Unrealized Gains	7	1
Unrealized Losses	(37)	(46)
Market Value	160,103	118,714
Level 2 \| U.S. Treasury securities included in short-term investments
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost	13,561	63,101
Unrealized Gains	4	4
Unrealized Losses	0	(1)
Market Value	13,565	63,104
Level 2 \| U.S. government agency securities included in cash and cash equivalents and short-term investments
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost	48,104	54,183
Unrealized Gains	9	10
Unrealized Losses	0	0
Market Value	$ 48,113	$ 54,193

Equity Incentive Plans and Stock-Based Compensation - Stock-Based Compensation Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	$ 0.2
Unrecognized compensation expense, weighted average period	1 year
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense, weighted average period	3 years 7 days
Unrecognized compensation cost	$ 30.0

Income Taxes - Additional Information (Details)	3 Months Ended
Income Taxes - Additional Information (Details)	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Valuation allowance as a percentage on total deferred tax assets	100.00%
Effective tax rate	0.00%

Related Party Transactions	3 Months Ended
Related Party Transactions	Mar. 31, 2021
Related Party Transactions [Abstract]
Related Party Transactions	14. Related Party Transactions In March 2020, the Company granted 26,573 stock options with a grant date fair value of $0.1 million to a partner at Third Rock Ventures, who is also serving as a non-employee director on the Company’s board of directors. The shares of common stock subject to these options vest 1/12th on the last day of each calendar quarter over a three-year period and commenced vesting upon completion of our IPO. Lastly, in order to vest at each calendar quarter end date, the stockholder must be providing continuous service to the Company through such vesting date. See Note 11 for additional information. From time to time, the Company makes charitable contributions to the University of California, San Francisco Foundation (the “UCSF Foundation”), which are directed to support research performed in the laboratories of two of the Company’s scientific founders. The Company made no charitable contributions to the UCSF Foundation during the three months ended March 31, 2021. The Company made charitable contributions to the UCSF Foundation in the amount of $0.7 million during the three months ended March 31, 2020. In October 2019, the Company entered into the Novartis Agreement with Novartis covering the development and commercialization of the Company's preclinical product candidate, PLN-1474 and up to three additional targets. Upon execution of the Agreement, Pliant also entered into a financing side letter with Novartis, whereby Novartis committed to provide up to $30.0 million in equity financing of which $20.0 million was provided for preferred stock as a part of a Series C equity offering and the remaining $10.0 million was completed in a private placement for common stock concurrently with the completion of the Company’s IPO. As of March 31, 2021 and December 31, 2020, Novartis owned approximately 6.0% and 6.1%, respectively, of our outstanding shares of common stock on a fully diluted basis. See Note 6 for additional information.

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($)
$ in Thousands

Total

Common Stock

Additional Paid-In Capital

Accumulated Other Comprehensive Gain (Loss)

Accumulated Deficit

Series A Redeemable Convertible Preferred Stock

Series B Redeemable Convertible Preferred Stock

Series C Redeemable Convertible Preferred Stock

Beginning balance (in shares) at Dec. 31, 2019

56,000,000

49,501,221

26,360,745

Beginning balance at Dec. 31, 2019

$ 62,468

$ 75,860

$ 47,947

Increase (Decrease) in Temporary Equity [Roll Forward]

Issuance of Series C redeemable preferred stock (in shares)

28,527,313

Issuance of Series C redeemable preferred stock, net of issuance costs

$ 52,038

Ending balance (in shares) at Mar. 31, 2020

56,000,000

49,501,221

54,888,058

Ending balance at Mar. 31, 2020

$ 62,468

$ 75,860

$ 99,985

Beginning balance (in shares) at Dec. 31, 2019

1,846,024

Beginning balance at Dec. 31, 2019

$ (76,295)

$ 1

$ 0

$ (1)

$ (76,295)

Increase (Decrease) in Stockholders' Equity [Roll Forward]

Vesting of restricted stock awards (in shares)

52,093

Vesting of restricted stock awards

Option exercises (in shares)

8,232

Option exercises

Stock-based compensation expense

425

Net unrealized gain on short-term investments

Net income (loss)

11,029

Ending balance (in shares) at Mar. 31, 2020

1,906,349

Ending balance at Mar. 31, 2020

$ (64,751)

$ 1

455

(65,266)

Beginning balance (in shares) at Dec. 31, 2020

Beginning balance at Dec. 31, 2020

$ 0

Ending balance (in shares) at Mar. 31, 2021

Ending balance at Mar. 31, 2021

$ 0

Beginning balance (in shares) at Dec. 31, 2020

35,552,795

Beginning balance at Dec. 31, 2020

$ 283,061

^[1]

$ 3

400,918

(32)

(117,828)

Increase (Decrease) in Stockholders' Equity [Roll Forward]

Vesting of restricted stock awards (in shares)

30,277

Vesting of restricted stock awards

Option exercises (in shares)

165,264

Option exercises

1,370

Stock-based compensation expense

2,578

Net unrealized gain on short-term investments

Net income (loss)

$ (22,856)

(22,856)

Ending balance (in shares) at Mar. 31, 2021

35,748,336

Ending balance at Mar. 31, 2021

$ 264,169

$ 3

$ 404,868

$ (18)

$ (140,684)

[1]	The condensed balance sheet as of December 31, 2020 has been derived from the audited financial statements as of that date.

Redeemable Convertible Preferred Stock - Additional Information (Details)	Mar. 31, 2021 stockClass shares
Temporary Equity [Line Items]
Number of classes of stock authorized \| stockClass	2
Redeemable Preferred Stock
Temporary Equity [Line Items]
Preferred stock, shares outstanding (in shares) \| shares	0

Condensed Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Common stock, par value (USD per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	300,000,000	300,000,000
Common stock, shares issued (in shares)	35,748,336	35,552,795
Common stock, shares outstanding (in shares)	35,748,336	35,552,795

Cover - shares	3 Months Ended
Cover - shares	Mar. 31, 2021	May 06, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Transition Report	false
Entity File Number	001-39303
Entity Registrant Name	PLIANT THERAPEUTICS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-4272481
Entity Address, Address Line One	260 Littlefield Avenue
Entity Address, City or Town	South San Francisco,
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	481-6770
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Trading Symbol	PLRX
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding (in shares)		35,894,459
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Central Index Key	0001746473
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1

Equity Incentive Plans and Stock-Based Compensation - Summary of Option Activity Under the 2015 Plan and 2020 Plan (Details) - 2015 Equity and 2020 Incentive Plan - Incentive Stock Options and Nonqualified Stock Options - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding, Beginning balance (in shares)	2,993,855
Options granted (in shares)	1,082,053
Number of Options, Exercised (in shares)	(105,734)
Number of Options, Forfeited (in shares)	(41,887)
Number of Options, Outstanding, Ending balance (in shares)	3,928,287	2,993,855
Number of Options, Exercisable (in shares)	856,283
Number of Options, Vested and expected to vest (in shares)	3,928,287
Share-Based Compensation Arrangement By Share-Based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Weighted- Average Exercise Price per Share, Outstanding, Beginning balance (USD per share)	$ 8.18
Weighted- Average Exercise Price per Share, Granted (USD per share)	27.11
Weighted- Average Exercise Price per Share, Exercised (USD per share)	5.28
Weighted- Average Exercise Price per Share, Forfeited (USD per share)	7.59
Weighted- Average Exercise Price per Share, Outstanding, Ending balance (USD per share)	13.30	$ 8.18
Weighted- Average Exercise Price per Share, Exercisable (USD per share)	5.24
Weighted- Average Exercise Price per Share, Vested and expected to vest (USD per share)	$ 13.30
Weighted- Average Remaining Contractual Term (in Years), Outstanding	8 years 11 months 19 days	8 years 10 months 2 days
Weighted- Average Remaining Contractual Term (in Years), Exercisable	8 years 3 months 21 days
Weighted- Average Remaining Contractual Term (in Years), Vested and expected to vest	8 years 11 months 19 days
Aggregate Intrinsic Value, Outstanding balance	$ 100,437	$ 43,890
Aggregate Intrinsic Value, Exercisable	29,194
Aggregate Intrinsic Value, Vested and expected to vest	$ 100,437

Regent of the University of California License Agreement (the "UC Agreement")	3 Months Ended
	Mar. 31, 2021
License Agreements [Abstract]
Regent of the University of California License Agreement (the "UC Agreement")	Regents of the University of California License Agreement (the "UC Agreement") In 2015, we entered into the UC Agreement to obtain an exclusive, worldwide license relating to the use of certain patents and technology relating to αvß1 compound in fibrosis indications. Pursuant to the UC Agreement, we made a $2.4 million milestone payment upon the close of our initial public offering in June 2020. Subsequently, we determined the licensed technology was no longer relevant to the development of our product candidates and, therefore, we exercised our right to terminate the UC Agreement which became effective in the first quarter of 2021. No further obligations or financial commitments survive the termination.

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2021

Commitments and Contingencies Disclosure [Abstract]

Schedule of Future Minimum Lease Payments

Future minimum lease payments under the Lease as of March 31, 2021 were as follows (in thousands):


Year ending December 31:	Operating Lease
2021 (remainder of the year)	$	1,529
2022	2,098
2023	2,171
2024	2,247
2025 and beyond	1,143
Total	$	9,188

Common Stock

3 Months Ended

Mar. 31, 2021

Equity [Abstract]

Common Stock

. Common Stock

The voting, dividend, and liquidation rights of the holders of the Company's common stock are subject to and qualified by the rights, powers, and preferences of the holders of preferred stock with preferential rights. As of March 31, 2021 and December 31, 2020, the Company was authorized to issue 300,000,000 shares of common stock at a par value of $0.0001 per share. The common stock has the following rights and privileges:

Voting

The holders of shares of common stock are entitled to one vote for each share of common stock held at any meeting of stockholders and at the time of any written action in lieu of a meeting.

Dividends

The holders of shares of common stock are entitled to receive dividends, when declared by the Company’s board of directors. Cash dividends may not be declared or paid to holders of shares of common stock until all unpaid dividends on preferred stock have been paid in accordance with their terms. No dividends have been declared or paid by the Company since its inception.

Liquidation

Subject to the preferential rights of holders of preferred stock then outstanding, the holders of shares of common stock are entitled to share ratably in the Company’s remaining assets available for distribution to its stockholders in the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company.

Shares reserved for future issuance


	As of March 31, 2021	As of December 31, 2020
Exercises of outstanding stock option awards	3,928,287	2,993,855
Shares of common stock available for future grants under the 2020 Equity Incentive Plan	4,340,045	3,644,459
Shares of common stock available for future issuance under the 2020 ESPP	640,234	700,000
Total shares reserved for future issuance	8,908,566	7,338,314

Equity Incentive Plans and Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 2,578	$ 425
Research and development expenses
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	1,170	164
General and administrative expenses
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	1,408	261
Restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	51	57
Stock options and ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 2,527	$ 368

Net (Loss)Income Per Share Attributable to Common Stockholders - Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator
Net income (loss)	$ (22,856)	$ 11,029
Less: undistributed earnings to preferred stockholders	0	(11,029)
Net loss attributable to common stockholders	$ (22,856)	$ 0
Denominator
Basic (in shares)	35,645,205	1,897,669
Diluted (in shares)	35,645,205	1,897,669
Net loss per share, attributable to common stockholders:
Basic (USD per share)	$ (0.64)	$ 0
Diluted (USD per share)	$ (0.64)	$ 0

Property and Equipment, net (Tables)

3 Months Ended

Mar. 31, 2021

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):


	As of March 31, 2021		As of December 31, 2020
Computer equipment and software	$	22	$	22
Laboratory equipment	6,921		6,540
Leasehold improvements	947		947
Construction-in-progress	223		300
Total property and equipment, gross	8,113		7,809
Less: Accumulated depreciation	(3,847)		(3,488)
Total property and equipment, net	$	4,266	$	4,321

Commitments and Contingencies

3 Months Ended

Mar. 31, 2021

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

13. Commitments and Contingencies

Purchase Commitments

The Company has contractual arrangements with research and development organizations and suppliers; however, these contracts are generally cancellable on 30 days’ notice and the obligations under these contracts are largely based on services performed.

License and Collaboration Agreements

Potential payments related to the Company’s license and research agreements, including milestone and royalty payments, are detailed in Notes 6 and 7.

Leases

In February 2018, the Company entered into a non-cancelable lease agreement (the “Lease”) for premises consisting of approximately 32,974 square feet located in South San Francisco, California (the “Premises”). The Company moved into the Premises in July 2018. The Premises is being used for the Company’s corporate headquarters and principal operating facility. The term of the Lease is eighty-four months, which commenced on July 1, 2018. Base rent was abated for the first two months of the lease term and thereafter is $0.2 million per month during the first year of the lease term, with specified annual increases thereafter. The Company paid a refundable security deposit of approximately $0.4 million, which is included in other non-current assets in the Balance Sheets at March 31, 2021 and December 31, 2020. The Company has the right to extend the lease term by seven years upon written notice not more than twelve months nor less than nine months prior to the expiration of the original lease term, with monthly payments equal to the “fair rental value” as defined in the Lease.

During the three months ended March 31, 2021 and 2020, rent expense, including common area maintenance expense, was $0.6 million.

Future minimum lease payments under the Lease as of March 31, 2021 were as follows (in thousands):


Year ending December 31:	Operating Lease
2021 (remainder of the year)	$	1,529
2022	2,098
2023	2,171
2024	2,247
2025 and beyond	1,143
Total	$	9,188

Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2021

Fair Value Disclosures [Abstract]

Summary of Cash and Cash Equivalents, Money Market Funds and Short-term Investments by Significant Investment Category

The following tables show the Company’s cash and cash equivalents, Money Market Funds and short-term investments by significant investment category as of March 31, 2021 and December 31, 2020 (in thousands):


	As of March 31, 2021
	Adjusted Cost		Unrealized Gains		Unrealized Losses		Market Value
Level 1:
Money Market Funds	$	22,056	$	—	$	—	$	22,056
Level 2:
U.S. Treasury securities included in short-term investments	13,561		4		—		13,565
U.S. government agency securities included in cash and cash equivalents and short-term investments	48,104		9		—		48,113
Corporate debt securities included in cash and cash equivalents and short-term investments	160,133		7		(37)		160,103
Total financial assets	$	243,854	$	20	$	(37)	$	243,837


	As of December 31, 2020
	Adjusted Cost		Unrealized Gains		Unrealized Losses		Market Value
Level 1:
Money Market Funds	$	27,686	$	—	$	—	$	27,686
Level 2:
U.S. Treasury securities included in short-term investments	63,101		4		(1)		63,104
U.S. government agency securities included in short-term investments	54,183		10		—		54,193
U.S. Corporate debt securities included in cash and cash equivalents and short-term investments	118,759		1		(46)		118,714
Total financial assets	$	263,729	$	15	$	(47)	$	263,697

Condensed Statements of Operations and Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Revenue — related party	$ 2,174	$ 28,938
Operating expenses:
Research and development	(18,527)	(13,919)
General and administrative	(6,566)	(4,011)
Total operating expenses	(25,093)	(17,930)
(Loss) income from operations	(22,919)	11,008
Interest and other income (expense), net	63	21
Net (loss) income	(22,856)	11,029
Less: Undistributed earnings to preferred stockholders	0	11,029
Net loss attributable to common stockholders	$ (22,856)	$ 0
Net loss per share, attributable to common stockholders:
Basic (USD per share)	$ (0.64)	$ 0
Diluted (USD per share)	$ (0.64)	$ 0
Shares used in computing net loss per share attributable to common stockholders:
Basic (in shares)	35,645,205	1,897,669
Diluted (in shares)	35,645,205	1,897,669
Comprehensive income (loss):
Net income (loss)	$ (22,856)	$ 11,029
Net unrealized gain (loss) on short-term investments	14	60
Total other comprehensive income	14	60
Comprehensive income (loss)	$ (22,842)	$ 11,089

Commitments and Contingencies - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended
	Feb. 28, 2018 USD ($) ft²	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Loss Contingencies [Line Items]
Contractual arrangements cancellation period		30 days
Base rent abated period	2 months
Rent expenses and common area maintenance charges		$ 0.6	$ 0.6
California
Loss Contingencies [Line Items]
Leased premises under non cancelable operating lease \| ft²	32,974
Lease term	84 months
Base rent abated amount	$ 0.2
Lease term extension period		7 years
Minimum \| California
Loss Contingencies [Line Items]
Lease term extension notice period		9 months
Maximum \| California
Loss Contingencies [Line Items]
Lease term extension notice period		12 months
Other Non-current Assets \| California
Loss Contingencies [Line Items]
Refundable security deposit		$ 0.4		$ 0.4

Organization and Description of Business	3 Months Ended
Organization and Description of Business	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Description of Business	1. Organization and Description of Business Pliant Therapeutics, Inc. (the “Company” or "Pliant" or “we” or “our” or “us”) is a clinical stage biopharmaceutical company focused on discovering and developing novel therapies for the treatment of fibrosis with an initial focus on treating fibrosis by inhibiting integrin-mediated activation of TGF-ß. Fibrosis refers to the abnormal thickening and scarring of connective tissue due to the production and deposition of excess collagen in the extra-cellular matrix. Fibrosis can occur in many different tissues including lung, liver, kidney, muscle, skin and the GI tract, and often causes severe and debilitating disease leading to organ failure. The Company is located in South San Francisco, California, and was incorporated in the state of Delaware in June 2015. Reverse Stock Split On May 22, 2020, the Company implemented a 1-for-7.15 reverse stock split of the Company’s common stock. Stockholders entitled to fractional shares as a result of the reverse stock split received a cash payment in lieu of receiving fractional shares. All share and per share data shown in the accompanying financial statements and related notes have been retroactively revised to reflect the reverse stock split. Shares of common stock underlying outstanding stock options and other equity instruments were proportionately reduced and the respective exercise prices, if applicable, were proportionately increased in accordance with the terms of the agreements governing such securities. Shares of common stock reserved for issuance upon the conversion of the Company’s convertible preferred stock were proportionately reduced and the respective conversion prices were proportionately increased. As of June 3, 2020, all outstanding preferred stock had been converted into common stock. Initial Public Offering In June 2020, the Company completed its initial public offering (the "IPO"), in which the Company issued and sold an aggregate of 10,350,000 shares of common stock, which consisted of 9,000,000 shares of common stock and 1,350,000 shares of common stock sold pursuant to the underwriters’ exercise of their option to purchase additional shares, at a public offering price of $16.00 per share. The aggregate net proceeds received by the Company from the offering were $148.3 million, net of underwriting discounts, commissions and offering expenses of $5.7 million. Upon the closing of the IPO, 160,389,279 shares of the Company’s outstanding convertible preferred stock were automatically converted to common stock on a 7.15:1 basis and the related carrying amount of $238.3 million was reclassified to common stock and additional paid-in capital within stockholders’ equity (deficit). Concurrent with the completion of the IPO, the Company also issued 625,000 shares of its common stock to Novartis Institutes for Biomedical Research, Inc. (“Novartis”), a strategic partner and existing stockholder of the Company, in a private placement at a price of $16.00 per share for proceeds of $10.0 million, which resulted in Novartis owning approximately 6.1% of the Company’s outstanding shares of common stock immediately after the IPO.

Equity Incentive Plans and Stock-Based Compensation - Incentive Stock Options and Nonqualified Stock Options Narrative (Details)	3 Months Ended
	Mar. 31, 2021 $ / shares shares
2015 Equity and 2020 Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Expiration period from date of grant	10 years
2020 Stock Options And Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period from date of grant	5 years
Incentive Stock Options and Nonqualified Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value of stock option (in shares) \| $ / shares	$ 17.62
Incentive Stock Options and Nonqualified Stock Options \| 2015 Equity and 2020 Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)	1,082,053
Incentive Stock Options and Nonqualified Stock Options \| 2020 Stock Options And Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)	1,082,053

Adimab Development and Option Agreement (the "Adimab Agreement") - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development expense	$ 18,527	$ 13,919
Adimab Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development expense	$ 28	$ 0

Equity Incentive Plans and Stock-Based Compensation - 2020 Employee Stock Purchase Plan Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended
	Jun. 30, 2020	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total shares reserved for future issuance (in shares)		8,908,566		7,338,314
Total stock-based compensation expense		$ 2,578,000	$ 425,000
2020 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total shares reserved for future issuance (in shares)				700,000
Common stock shares outstanding percentage	1.00%
Percentage of payroll deductions		15.00%
Employee stock purchase plan offering period		6 months
Common stock fair market value percentage		85.00%
Total stock-based compensation expense		$ 300,000	$ 0
Number of common stock issued (in shares)		59,766

Novartis Collaboration and License Agreement ("the Novartis Agreement")	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Novartis Collaboration and License Agreement ("the Novartis Agreement")	6. Novartis Collaboration and License Agreement (the "Novartis Agreement") In 2019, we entered into the Novartis Agreement for the development and commercialization of our preclinical product candidate, PLN-1474 and up to three additional integrin research targets. PLN-1474 is an internally discovered small molecule selective inhibitor of integrin αvß1, currently being developed for the treatment of liver fibrosis associated with nonalcoholic steatohepatitis (“NASH”). Pursuant to the agreement, we received an upfront, non-refundable license fee of $50.0 million and were eligible to receive additional payments of $416.0 million contingent upon achievement of specified research, development, regulatory and commercial events and royalties on world-wide net sales thereafter. Additionally, Novartis is funding up to $19.6 million associated with research and development services for PLN-1474 and up to $13.4 million for research and development services on the integrin research targets. To date, we have received $25.0 million in contingent payments and $391.0 million remain eligible for achievement. No contingent payments were recognized during the first quarter of 2021 and one contingent payment of $25.0 million was recognized in the first quarter of 2020. Revenues for research and development services for the three months ended March 31, 2021 and 2020 were $2.2 million and $3.9 million, respectively.

Summary of Significant Accounting Policies	3 Months Ended
Summary of Significant Accounting Policies	Mar. 31, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Certain prior year reported amounts have been reclassified to conform with the current period presentation. The accompanying condensed balance sheet as of March 31, 2021, condensed statements of operations and comprehensive income (loss), condensed statements of cash flows, and the condensed statements of redeemable convertible preferred stock and stockholders’ equity (deficit) for the three months ended March 31, 2021 and 2020, are unaudited. The balance sheet as of December 31, 2020 was derived from audited financial statements as of and for the year ended December 31, 2020. The unaudited interim financial statements have been prepared on the same basis as the audited annual financial statements as of and for the year ended December 31, 2020, and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of March 31, 2021, and the results of its operations and its cash flows for the three months ended March 31, 2021 and 2020. The financial data and other information disclosed in these notes related to the three months ended March 31, 2021 and 2020, are also unaudited. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, and expenses as well as the disclosure of contingent assets and liabilities as of and during the reporting period. The Company bases its estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. Significant estimates and assumptions reflected in these financial statements include, but are not limited to stock-based compensation expense and accruals for research and development costs. The Company assesses estimates on an ongoing basis, however, actual results could materially differ from those estimates. Significant Accounting Policies There have been no significant changes to the accounting policies during the three months ended March 31, 2021, as compared to the significant accounting policies described in Note 2 of the "Notes to the Financial Statements" in the Company's audited financial statements included in its Annual Report on Form 10-K for the fiscal year ended December 31, 2020. Recently Issued Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) No. 2016-2, Leases (“Topic 842”), which requires an entity to recognize assets and liabilities arising from a lease for both financing and operating leases. For public entities, ASU No. 2016-2 is effective for fiscal years beginning after December 15, 2018. The ASU will also require new qualitative and quantitative disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the Jumpstart Our Business Startups Act of 2012 (the "JOBS Act"), ASU No. 2016-2 is effective for the Company in the fiscal years beginning after December 15, 2021, and interim periods within fiscal years beginning after December 15, 2022, with early adoption permitted. The Company is currently in the process of evaluating the impact of the adoption of ASU No. 2016-2 on the Company’s financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The new standard amends guidance on measuring and reporting credit losses for financial assets held at amortized cost basis, including accounts receivable and investments classified as available for sale, such as our debt securities. This ASU requires a new forward-looking model based on expected credit losses rather than the current one based on incurred losses. In November 2019, the FASB issued ASU 2019-10, which deferred the effective date for certain ASUs including ASU 2016-13. This standard is effective for the Company’s fiscal year beginning after December 15, 2022. Early adoption is permitted for all entities. The Company is currently in the process of evaluating the impact of the adoption of ASU 2016-13 on the Company’s financial statements.

Redeemable Convertible Preferred Stock	3 Months Ended
Redeemable Convertible Preferred Stock	Mar. 31, 2021
Temporary Equity Disclosure [Abstract]
Redeemable Convertible Preferred Stock	. Redeemable Convertible Preferred StockUnder the Company’s Amended and Restated Certificate of Incorporation (“Certificate of Incorporation”), the Company is authorized to issue two classes of shares: preferred and common stock. The preferred stock may be issued in series, and the Company’s board of directors is authorized to determine the rights, preferences, and terms of each series. These rights preferences and terms could include dividend rights, conversion rights, voting rights, terms of redemptions, liquidation preferences and sinking fund terms. As a result of the IPO in June 2020, in which all then outstanding convertible preferred stock was converted into shares of common stock of the Company, there is no outstanding redeemable convertible preferred stock as of March 31, 2021.

Financial Instruments

3 Months Ended

Mar. 31, 2021

Fair Value Disclosures [Abstract]

Financial Instruments

3. Financial Instruments

The Company’s short-term investments consist of U.S. Treasury securities, U.S. Government agency securities and highly rated, investment-grade corporate debt securities with original maturities beyond three months at the date of purchase. The Company has classified and accounted for its short-term investments as available-for-sale securities as the Company may sell these securities at any time even prior to maturity and such investments represent cash available for current operations. As a result, short-term investments may include securities with maturities beyond twelve months that are classified within current assets in the Balance Sheets. The Company’s short-term investments classified as available-for-sale are carried at fair market value with unrealized losses or income recognized in other comprehensive income (loss).

The Company’s cash equivalent Money Market Funds are classified as Level 1 because they are valued using quoted market prices. The fair value of the Company’s U.S. Treasury securities, U.S. government agency securities and corporate debt securities are classified as Level 2 because they are valued using observable inputs to quoted market prices, benchmark yields, reported trades, broker/dealer quotes or alternative pricing sources with reasonable levels of price transparency and include U.S. government agency securities, U.S. Treasury securities and corporate debt securities.

There were no Level 3 assets or liabilities as of March 31, 2021 and as of December 31, 2020.


	As of March 31, 2021
	Adjusted Cost		Unrealized Gains		Unrealized Losses		Market Value
Level 1:
Money Market Funds	$	22,056	$	—	$	—	$	22,056
Level 2:
U.S. Treasury securities included in short-term investments	13,561		4		—		13,565
U.S. government agency securities included in cash and cash equivalents and short-term investments	48,104		9		—		48,113
Corporate debt securities included in cash and cash equivalents and short-term investments	160,133		7		(37)		160,103
Total financial assets	$	243,854	$	20	$	(37)	$	243,837


	As of December 31, 2020
	Adjusted Cost		Unrealized Gains		Unrealized Losses		Market Value
Level 1:
Money Market Funds	$	27,686	$	—	$	—	$	27,686
Level 2:
U.S. Treasury securities included in short-term investments	63,101		4		(1)		63,104
U.S. government agency securities included in short-term investments	54,183		10		—		54,193
U.S. Corporate debt securities included in cash and cash equivalents and short-term investments	118,759		1		(46)		118,714
Total financial assets	$	263,729	$	15	$	(47)	$	263,697

The Company may sell certain of its short-term securities prior to their stated maturities for reasons including, but not limited to, managing liquidity, credit risk, duration and asset allocation.

There were no liabilities measured at fair value on a recurring basis as of March 31, 2021 and December 31, 2020. There have been no transfers between fair value measurement levels during the three months ended March 31, 2021 and 2020. In addition, there were no assets or liabilities measured at fair value on a non-recurring basis as of March 31, 2021 and December 31, 2020.

As of March 31, 2021, the Company had not recorded any impairment related to other-than-temporary declines in the fair value of short-term investments. Short-term investments are considered impaired when a decline in fair value is judged to be other-than-temporary. The Company consults with its investment managers and considers available quantitative and qualitative evidence in evaluating potential impairment of its short-term investments on a quarterly basis. If the cost of an individual investment exceeds its fair value, the Company evaluates, among other factors, general market conditions, the duration and extent to which the fair value is less than cost and its intent and ability to hold the investment.

The Company records interest income and accretion income earned on Money Market Funds and U.S. Treasury, U.S. government agency and corporate debt securities to interest and other income (expense), net in its condensed statement of operations and comprehensive income (loss).

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Related Party Transactions - Narrative (Details)	1 Months Ended		3 Months Ended
Related Party Transactions - Narrative (Details)	Mar. 31, 2020 USD ($) shares	Oct. 31, 2019 USD ($) stockClass	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020
Novartis Agreement \| Novartis
Related Party Transaction [Line Items]
Number of Integrin research targets \| stockClass		3
Committed to provide equity financing (up to)		$ 30,000,000.0
Percentage of outstanding equity on fully diluted basis			6.00%		6.10%
IPO \| Novartis Agreement \| Novartis
Related Party Transaction [Line Items]
Committed to provide equity financing (up to)		10,000,000.0
Series C Redeemable Convertible Preferred Stock \| Novartis Agreement \| Novartis
Related Party Transaction [Line Items]
Committed to provide equity financing (up to)		$ 20,000,000.0
Third Rock Ventures \| Non-employee Director \| Stock Options
Related Party Transaction [Line Items]
Options granted (in shares) \| shares	26,573
Grant date fair value of stock option	$ 100,000
Vesting rights, percentage			8.3333%
Vesting period			3 years
UCSF Foundation
Related Party Transaction [Line Items]
Transaction price of Novartis agreement			$ 0	$ 700,000

Regent of the University of California License Agreement (the "UC Agreement") - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
License Agreements \| UC Regents
License Agreements [Line Items]
Milestone and annual royalty paid	$ 2.4

Equity Incentive Plans and Stock-Based Compensation - Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for Stock Option Awards (Details) - Incentive Stock Options and Nonqualified Stock Options - $ / shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	74.95%	72.10%
Expected volatility, maximum	75.72%	77.50%
Risk-free interest rate, minimum	0.61%	0.52%
Risk-free interest rate, maximum	1.06%	0.82%
Expected dividend	0.00%	0.00%
Underlying common stock fair value (USD per share)		$ 6.22
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 9 months 7 days	5 years 4 months 17 days
Underlying common stock fair value (USD per share)	$ 26.13
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 9 months
Underlying common stock fair value (USD per share)	$ 38.23

Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
Condensed Statements of Cash Flows - USD ($) $ in Thousands	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities
Net income (loss)	$ (22,856)	$ 11,029
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Depreciation expense	359	312
Stock-based compensation expense	2,578	425
Other	271	0
Changes in operating assets and liabilities:
Tax credit receivable	0	72
Accounts receivable	7,105	(24,710)
Prepaid expenses and other current assets	318	(883)
Other non-current assets	0	233
Accounts payable	665	1,176
Accrued liabilities	(2,092)	(424)
Other long-term liabilities	(13)	4
Net cash used in operating activities	(13,665)	(12,766)
Cash flows from investing activities
Purchase of short-term investments	(64,806)	(11,008)
Maturity of short-term investments	77,280	0
Accretion of short-term investments	0	(29)
Purchase of property and equipment	(242)	(336)
Net cash provided by/(used in) investing activities	12,232	(11,373)
Cash flows from financing activities
Proceeds from exercise of stock options	1,370	26
Payment of deferred offering costs	0	(364)
Net cash provided by financing activities	1,370	51,700
Net (decrease)/increase in cash and cash equivalents	(63)	27,561
Cash and cash equivalents at beginning of period	50,882	85,807
Cash and cash equivalents at end of period	50,819	113,368
Supplemental disclosures of noncash investing and financing activities:
Purchase of property and equipment in accounts payable and accrued liabilities	251	257
Reclassification of restricted stock awards from liabilities to common stock upon vesting	2	4
Deferred offering costs in accounts payable and accrued liabilities	0	792
Net unrealized gain on short-term investments	14	60
Series C Preferred Stock
Cash flows from financing activities
Proceeds from issuance of Series C preferred stock, net of issuance costs	$ 0	$ 52,038

Common Stock (Tables)

3 Months Ended

Mar. 31, 2021

Equity [Abstract]

Schedule of Shares Reserved for Future Issuance

Shares reserved for future issuance


	As of March 31, 2021	As of December 31, 2020
Exercises of outstanding stock option awards	3,928,287	2,993,855
Shares of common stock available for future grants under the 2020 Equity Incentive Plan	4,340,045	3,644,459
Shares of common stock available for future issuance under the 2020 ESPP	640,234	700,000
Total shares reserved for future issuance	8,908,566	7,338,314

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Certain prior year reported amounts have been reclassified to conform with the current period presentation. The accompanying condensed balance sheet as of March 31, 2021, condensed statements of operations and comprehensive income (loss), condensed statements of cash flows, and the condensed statements of redeemable convertible preferred stock and stockholders’ equity (deficit) for the three months ended March 31, 2021 and 2020, are unaudited. The balance sheet as of December 31, 2020 was derived from audited financial statements as of and for the year ended December 31, 2020. The unaudited interim financial statements have been prepared on the same basis as the audited annual financial statements as of and for the year ended December 31, 2020, and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of March 31, 2021, and the results of its operations and its cash flows for the three months ended March 31, 2021 and 2020. The financial data and other information disclosed in these notes related to the three months ended March 31, 2021 and 2020, are also unaudited.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, and expenses as well as the disclosure of contingent assets and liabilities as of and during the reporting period. The Company bases its estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. Significant estimates and assumptions reflected in these financial statements include, but are not limited to stock-based compensation expense and accruals for research and development costs. The Company assesses estimates on an ongoing basis, however, actual results could materially differ from those estimates.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) No. 2016-2, Leases (“Topic 842”), which requires an entity to recognize assets and liabilities arising from a lease for both financing and operating leases. For public entities, ASU No. 2016-2 is effective for fiscal years beginning after December 15, 2018. The ASU will also require new qualitative and quantitative disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the Jumpstart Our Business Startups Act of 2012 (the "JOBS Act"), ASU No. 2016-2 is effective for the Company in the fiscal years beginning after December 15, 2021, and interim periods within fiscal years beginning after December 15, 2022, with early adoption permitted. The Company is currently in the process of evaluating the impact of the adoption of ASU No. 2016-2 on the Company’s financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The new standard amends guidance on measuring and reporting credit losses for financial assets held at amortized cost basis, including accounts receivable and investments classified as available for sale, such as our debt securities. This ASU requires a new forward-looking model based on expected credit losses rather than the current one based on incurred losses. In November 2019, the FASB issued ASU 2019-10, which deferred the effective date for certain ASUs including ASU 2016-13. This standard is effective for the Company’s fiscal year beginning after December 15, 2022. Early adoption is permitted for all entities. The Company is currently in the process of evaluating the impact of the adoption of ASU 2016-13 on the Company’s financial statements.

Defined Contribution Plan - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Retirement Benefits [Abstract]
Payment made under defined contribution plan	$ 0.2	$ 0.2

Net (Loss)Income Per Share Attributable to Common Stockholders - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted net loss per share	4,028,727	25,277,965
Exercises of outstanding stock option awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted net loss per share	3,928,287	2,603,744
Restricted stock awards granted and not purchased
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted net loss per share	4,195	4,195
Unvested restricted shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted net loss per share	96,245	237,997
Redeemable convertible preferred stock (on an as-converted basis)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted net loss per share	0	22,432,029

Common Stock - Schedule of Shares Reserved for Future Issuance (Details) - shares	Mar. 31, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Total shares reserved for future issuance (in shares)	8,908,566	7,338,314
Shares of common stock available for future grants under the 2020 Equity Incentive Plan
Class of Stock [Line Items]
Total shares reserved for future issuance (in shares)	4,340,045	3,644,459
Shares of common stock available for future issuance under the 2020 ESPP
Class of Stock [Line Items]
Total shares reserved for future issuance (in shares)	640,234	700,000
Exercises of outstanding stock option awards
Class of Stock [Line Items]
Total shares reserved for future issuance (in shares)	3,928,287	2,993,855

Equity Incentive Plans and Stock-Based Compensation - Equity Incentive Plan Narrative (Details) - shares	1 Months Ended					3 Months Ended
	May 31, 2020	Mar. 31, 2020	Jan. 31, 2019	Jul. 31, 2018	Aug. 31, 2015	Mar. 31, 2021	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total shares reserved for future issuance (in shares)						8,908,566	7,338,314
2015 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares authorized for future grant (in shares)		1,400,000	400,000	1,000,000.0
Exercise price as a percentage of estimated grant date fair value of shares					10.00%
Expiration period from date of grant					5 years
2020 Stock Options And Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares outstanding percentage	5.00%
Total shares reserved for future issuance (in shares)						4,300,000
Exercise price as a percentage of estimated grant date fair value of shares						10.00%
Expiration period from date of grant						5 years
Maximum \| 2015 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for future grant (in shares)					1,500,000
Percent of estimated fair value of shares					100.00%
Options under plan may be granted for periods					10 years
Exercise price as a percentage of estimated grant date fair value of shares					110.00%
Maximum \| 2020 Stock Options And Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for future grant (in shares)	4,200,000
Options under plan may be granted for periods						10 years
Exercise price as a percentage of estimated grant date fair value of shares						110.00%

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	12. Income Taxes For the three months ended March 31, 2021 and 2020, the Company did not record an income tax provision. The Company will continue to maintain a 100% valuation allowance on total deferred tax assets. The Company believes it is more likely than not that the related deferred tax asset will not be realized. As a result, the Company’s effective tax rate will remain at 0% because no items that are either estimated or discrete items would impact the tax provision.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Organization and Description of Business - Narrative (Detail) $ / shares in Units, $ in Millions		1 Months Ended
	May 22, 2020	Jun. 30, 2020 USD ($) $ / shares shares	Mar. 31, 2021 shares	Dec. 31, 2020 shares
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reverse stock split, conversion ratio	0.13986014
Common stock, shares issued (in shares)			35,748,336	35,552,795
Initial Public Offering
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reverse stock split, conversion ratio		7.15
Initial public offering, shares issued (in shares)		10,350,000
Share price of shares issued in initial public offering (USD per share) \| $ / shares		$ 16.00
Proceeds from the initial public offering \| $		$ 148.3
Underwriting discounts, commissions and offering expenses \| $		$ 5.7
Convertible preferred stock converted into common shares (in shares)		160,389,279
Reclassification of convertible preferred stock into additional paid in capital upon initial public offering \| $		$ 238.3
Initial Public Offering \| Underwriters
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Initial public offering, shares issued (in shares)		1,350,000
Initial Public Offering \| Common Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Initial public offering, shares issued (in shares)		9,000,000
Private Placement \| Novartis
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock, shares issued (in shares)		625,000
Share price (USD per share) \| $ / shares		$ 16.00
Proceeds from issuance of common stock \| $		$ 10.0
Private Placement \| Pliant Therapeutics, Inc. \| Novartis
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percentage of noncontrolling ownership		6.10%

Property and Equipment, net - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 359	$ 312